1. Field of the Invention
The present invention relates to a focus error detecting device for detecting focus error of a light beam and an optical pickup device having the focus error detecting device.
2. Description of the Related Art
As information recording medium with which information recording or information reproduction is optically performed, an optical disc such as a CD (Compact disc) and a DVD (Digital Versatile Disc) are known. Further, the development of a larger capacity recording medium is pursued, for example, a Blu-ray disc and an apparatus therefor using a blue-violet semiconductor laser since the laser has a shorter wavelength, and thus has an advantage of decreasing a beam spot thereof.
The research and development of a higher precision optical pickup device is proceeding in accordance with the progress of the optical disc as described above. In such an optical pickup device, a light beam such as a laser beam is applied to an optical disc when recording or reproduction is performed. It is important to focus the light beam on a position to be recorded or reproduced with precision while correcting aberration produced in the reflected light from the optical disc. For example, the aberration changes during recording or reproduction of the optical disc, since a cover layer thickness of an optical disc generally has in-plane distribution.
Some conventional aberration correcting devices for correcting such aberration use a collimator lens for collimating a light beam or a beam expander for changing a beam diameter of the light beam. For example, there is one disclosed in Japanese Patent Application Kokai No. 10-106012. These aberration correcting devices are for correcting spherical aberration of the light beam, which is produced by the optical disc, by moving the collimator lens or the beam expander along an optical axis of the light beam. Alternatively, study on a light beam shaping optical element for shaping a form of a light beam emitted from a light source etc., or study on a semiconductor laser for causing an aspect ratio of the emitted light beam to approach one etc. is proceeding. When such light beam as described above is used, it becomes easier to provide an optical pickup device for correcting spherical aberration by moving the collimator lens along the optical axis.
However, there is a problem that high order spherical aberration remains uncorrected when correcting the spherical aberration by using a group of lenses. Therefore, distortion of a spot is produced on a detecting element by residual ray aberration even when the spherical aberration is corrected on the disc surface. As a result, focus servo does not act on the best image point despite that the spherical aberration is corrected to result in occurrence of defocusing. Accordingly, there causes a problem in recording and reproduction operation due to the occurrence of defocusing.